Speed of individual photons in a vacuum?

[Son Goku]

I would just note that due to the Reeh-Schlieder theorem there are no states of finite support with a well-defined particle number. This is what ultimately prevents the definition of a position operator for any type of particle in QFT.
For massive particles there is the Newton-Wigner operator which was a proposed attempt at a position operator, but these don't even commute at space like distances, nor are they elements of the observable algebra. They aren't true observables.
So for all particles in QFT there is really just the probability to excite local probes and no sensible notion of position.

 

[Christian Thom]

TL;DR Summary: Does speed of individual photons in a vacuum vary?

Is there experimental evidence that confirms that the speed of individual photons in a vacuum never varies, even slightly?

Thanks in advance.

I worked in a lab that measured the distance Earth to moon with laser pulses, only few photons returned due to the distance, but the assumption that they move at the speed of light was never questioned and results were coherent with the expected movement of the moon, that is of course in the range of accuracy of a few mm with respect to 400000 km.